Several years' study of T-cell and B-cell differentiation has led to the description of discrete sets of T-cells and B-cells. These can be distinguished by allotypic markers that characterize the surface phenotypes of the sets. The sets differ also in their functional properties, and in the specificity of inducers that move them from one compartment of differentiation to the next. Another marker of differentiation for cells in the lymphocyte series is an enzyme called terminal deoxynucleotidyl transferase (TdT). This is an unusual DNA polymerase that requires a primer but no template, and is demonstrable only in the bone marrow and thymus, and certain leukemias of avian and mammalian species. It has been proposed by Baltimore that this enzyme may be responsible for generating lymphocyte receptor diversity, by inserting random nucleotide bases into a selectively nicked variable region gene. We propose to ascertain the chronology and site of terminal transferase in relation to the distinct divergent and sequential differentiative steps in normal lymphocyte differentiation, by relating the presence or absence of the enzyme to particular cell sets, and to tumors representing those cell sets. In further exploration of lymphocyte differentiation, and of the properties of lymphocyte sets, we propose: (1) To ascertain what differentiative step, i.e., what sets, can confer susceptibility to oncogenic transformation by Abelson virus and (2) to ascertain what potentialities for differentiation are retained by such a transformed set, especially in relation to commitment and expression of the set program, and we intend to explore the possibility of obtaining cloned transformed pro-thymocytes and pro-B-cells that could be used in the further analysis of lymphocyte inductions.